The audience ______ to see who would win the gold medal.

A．took a deep breath

B．held their breath

C．lost their breath

D．was out of breath

决定X射线管靶材适用性的两个因素是（）

A.拉伸强度和倔强强度

B.硬度和磁导度

C.电阻和抗氧化性能

D.原子序数和熔点